Scala Play: List to Json-Array

Question

I've got a List which holds some Personalization-objects. The latter is defined like this:

  sealed case class Personalization(firstname: String, lastname: String, keycardId: String)

I need to map this list to a Json-Array structure which has to look like this:

"personalization": [
    {
      "id": "firstname",
      "value": "John"
    },
    {
      "id": "lastname",
      "value": "Doe"
    }...

I am struggling with the part of mapping the field information to id/value pairs. Normally, I would create a play.api.libs.json.Format out of the Personalization class and let it map automatically -> Json.format[Personalization] - but this time, I need to create an array where an entry can hold n attributes.

Therefore I am asking for advice, if there is a possibility to use the Scala Play-Framework? Any input is much appreciated, thank you!

Answer 1

Writing as such JSON representation is not quite complex, using Writes.transform.

import play.api.libs.json._

case class Personalization(firstname: String, lastname: String, keycardId: String) // No need to seal a case class

implicit def writes: Writes[Personalization] = {
  val tx: JsValue => JsValue = {
    case JsObject(fields) => Json.toJson(fields.map {
      case (k, v) => Json.obj("id" -> k, "value" -> v)
    })

    case jsUnexpected => jsUnexpected // doesn't happen with OWrites
  }

  Json.writes[Personalization].transform(tx)
}

Which can be tested as bellow.

val personalization = Personalization(
  firstname = "First",
  lastname = "Last",
  keycardId = "Foo")

val jsonRepr = Json.toJson(personalization)
// => [{"id":"firstname","value":"First"},{"id":"lastname","value":"Last"},{"id":"keycardId","value":"Foo"}]

Reading is a little bit tricky:

implicit def reads: Reads[Personalization] = {
  type Field = (String, Json.JsValueWrapper)

  val fieldReads = Reads.seq(Reads[Field] { js =>
    for {
      id <- (js \ "id").validate[String]
      v <- (js \ "value").validate[JsValue]
    } yield id -> v
  })

  val underlying = Json.reads[Personalization]

  Reads[Personalization] { js =>
    js.validate(fieldReads).flatMap { fields =>
      Json.obj(fields: _*).validate(underlying)
    }
  }
}

Which can be tested as bellow.

Json.parse("""[
  {"id":"firstname","value":"First"},
  {"id":"lastname","value":"Last"},
  {"id":"keycardId","value":"Foo"}
]""").validate[Personalization]
// => JsSuccess(Personalization(First,Last,Foo),)

Note that is approach can be used for any case class format.